On Tuesday, Democrat Danny O'Connor fought Republican Troy Balderson to what's essentially a draw in a special election for Ohio's 12th congressional district. In 2016, the district went for President Trump by 11 percentage points and elected Rep. Pat Tiberi (R) by 37 points. Balderson will presumably eke out a victory after the 8,500 absentee and provisional ballots are processed. Indeed, he has already claimed victory, and a win is a win.

But the close race in the deep-red Ohio district "reinforced a trend that has been developing for more than a year," The Washington Post points out: "Democrats are routinely beating their 2016 performance by double digits — putting the House and perhaps even the Senate within their grasp in November." Here are four reasons Tuesday's elections were pretty ominous for Republicans:

1. There are a lot of GOP House districts more vulnerable than Ohio's 12th

In Tuesday's election, Ohio's 12th district "looks to have shifted by about 13 points towards Democrats relative to its partisan lean (how we would expect it to vote in a neutral political environment)," says FiveThirtyEight's Micah Cohen. "That's just about in line with Democrats' average overperformance in these elections in the Trump era." And that isn't great news for the GOP, as Cook Political Report's Dave Wasserman explains:

There are 68 R-held House seats less Republican than #OH12, per @CookPolitical PVI, and 119 less Republican than the #PA18 seat Dems won in March. Dems need to flip 23 to win House.

If Democrats continue to outperform by double-digits in November, that "blue wave" will crash over House Republicans.

2. Trump isn't a panacea for struggling Republicans

Trump's campaign stop in Ohio, and millions of dollars from national Republicans, may have pushed Balderson over the top, but "with the battlefield expanding to dozens of House districts and a handful of key Senate races, those particular advantages stand to be diluted," the Post says, and the "fundamentals" this year tend to favor the Democrats. Plus, midterms tend to be a referendum on the party in power — and Trump is historically unpopular, CNBC's John Harwood notes. "The 54 percent of Americans who disapprove of his job performance in this week's Gallup poll exceeds the disapproval at a similar point for any of the previous six presidents, beginning with Jimmy Carter. Moreover, Trump has reshaped the GOP in ways that leave the party dependent on greater support from a shrinking segment of the population."

Even after Alexandria Ocasio-Cortez stunned national Democrats by unseating the No. 4 House Democrat in June, "signs of a Tea Party-like movement in the Democratic Party that would throw winnable races to far-left candidates appear to be fading," David Weigel reports at The Washington Post. On Tuesday, establishment Democratic Reps. Adam Smith (Wash.) and William Lacy Clay (Mo.) easily fended off Ocasio-Cortez-backed challengers, and in Michigan, Gretchen Whitmer handily beat a candidate supported by Sen. Bernie Sanders (I-Vt.). "In suburban House districts across the Midwest, left-wing candidates lost to Democrats backed by party leaders, abortion rights groups, and labor unions," Weigel notes, and "there's little evidence that the argument inside the Democratic Party has hurt its November prospects." He had some numbers to back that up:

In the generic ballot average tabulated by FiveThirtyEight, Democrats held a 5.7-percentage-point lead on June 26. Six weeks later — after plenty of discussion of whether the party was lurching too far left — the lead had inched up to 8 points. Democrats were also heading to the polls in larger numbers than Republicans. By the end of June, 13.6 million votes had been cast in Democratic primaries, compared with 10.4 million in Republican primaries. Since 2014, the last midterm year, Republican turnout had grown by 24 percent; Democratic turnout had surged by 84 percent. That trend appeared to continue on Tuesday, with Democrats running far ahead of their turnout four years earlier in Kansas and Michigan. [The Washington Post]

4. Republicans still can't seem to find a winning message

"Virtually every Democratic special-election candidate has run on health care and economic fairness — not taking direct aim at Trump and his administration as much as a Republican policy agenda that they say favors the rich and well-connected over ordinary Americans," the Post notes. "Republicans, meanwhile, have flitted from issue to issue seeking to promote their own candidates and disqualify Democrats," as national Republicans tried to do with O'Connor. Republican "hopes of riding last year's GOP tax cuts to victory have largely faded along with the tax bill's popularity," and the sound defeat of a GOP-championed measure to make Missouri a "right to work" state shows the GOP's anti-union rhetoric may not be a political winner this year.

It is certainly possible that Republicans will find a way to turn this around. They'll "have to do something really significant in September if they want to keep the House in November," GOP pollster Frank Luntz tweeted Tuesday night. But "the polling and race ratings might be understating the Democrats' advantage," Harry Enten writes at CNN. "In other words, Democrats may actually be in slightly better shape than we think."